06_Equilibrium_Calculations-page2 - Determine the...

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Unformatted text preview: Determine the concentration of silver ion when excess AgCl is added to 0.1M NaBr. AgCl(s) Ag+ + Cl- We are adding a relative insoluble material to a solution that contains an anion that also produces a precipitate with silver. AgBr(s) Ag+ + Br- KSP AgBr = [Ag+][Br-] = 4.9x10-13 We know that [Na+] = 0.1 M KSP AgCl = [Ag+][Cl-] = 1.8x10-10 We want to know [Ag+] Its reasonable to assume that as silver enters the system, some will precipitate as AgBr. Lets write down the equilibria involved. OK, we have 4 species: Na+, Ag+, Cl- and BrWe know that Na+ = 0.1M since there is no reason for it to change. We have three unknowns but only two equations so far. We need to consider mass or charge balances. Lets see what we know about the system. We know that [Na+] = 0.1 so 0.1 + [Ag+] = [Cl-] + [Br-] So, we now have three equations and three unknowns -- we’re ready to solve this thing. We now need to express a single equation in terms of one unknown. We’ll use our mass balance First, every silver that enters the solution brings a chloride with it. However, some silver is expected to precipitate as AgBr. The total charge of the system must equal 0. so [ Na+ ] + [ Ag+] = [ Cl- ] + [ Br- ] 0.1 + [Ag+] = [Cl-] + [Br-] Br- and Cl- can be expressed in terms of silver using our KSP expressions: [Br-] = KSP AgBr / [Ag+] [Cl-] = KSP AgCl / [Ag+] 0.1 + [Ag+]= KSPAgBr / [Ag+] + KSPAgCl / [Ag+] ...
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